The wrongdoing they're looking for, which people have been pointing out for a while, is whether or not JP Morgan's London traders etc. hid or manipulated information that could have revealed this loss to regulators. JP Morgan says they're cooperating with the probe.

Basically, they're looking for a cover-up.

Just today, Reuters broke the news that a fourth trader in the London CIO is being implicated in the trading loss. His name is Julien Grout and he worked under Bruno Iksil (the London Whale). Grout still works at JPM, and he's hired a lawyer.